The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from two-wheel drive to four-wheel drive when at least one of the wheels of the vehicle slips.
In a conventional four-wheel drive vehicle, a power transmission system for the two-wheel drive is selectively converted to the four-wheel drive by engaging a clutch which is manually operated by a select lever.
When the vehicle travels on slippery, icy or snowy road by two-wheel drive, the transmission system should be changed to the four-wheel drive in order to prevent wheels from slipping. If the changing operation is done after the slipping of wheels because of the driver's misjudgement of the slipping, the slipping cannot be stopped or reduced. Therefore, it is necessary to change the transmission system to the four-wheel drive at the first stage of the slipping.
To meet such a requirement, an automatic control system for the four-wheel drive has been proposed in Japanese Patent Application No. 56-155857 (laid-open publication No. 58-56924 laid open on Apr. 4, 1983) which was filed by the assignee of this application; the publication date is prior to the application date but after the priority date of the present invention and this may not be prior art and is not admitted as prior art. Slipping of wheels can be detected by sensing excessive angular acceleration of drive wheels. Accordingly, the system is provided with a slip detecting circuit which produces a clutch signal when the angular acceleration of the drive wheels exceeds a predetermined reference value. The clutch signal causes a clutch to engage, so that the two-wheel drive is automatically changed to the four-wheel drive.
It will be noted that rapid starting and rapid acceleration also lead to angular acceleration. In order to distinguish this from the angular acceleration during slipping of wheels, the reference value is set at a value higher than the highest value caused by the rapid acceleration at the largest torque which occurs under non-slipping conditions.
In this connection, it should be noted that the driving force decreases as vehicle speed increases. FIG. 5 shows the relationship between the vehicle speed and the driving force, in which a is a driving force curve at the first speed of a transmission, b, c and d are driving force curves at the second, third and fourth speeds, respectively, and e(V) which is a function of vehicle speed driving force curve. It will be understood that angular acceleration at slipping decreases as the tangential driving force curve e decreases, namely as the vehicle speed increases. Therefore, if the reference value for detecting the slipping is set at a high value, slipping at a high speed driving cannot be detected. Thus, the reference value should be changed according to the change of the vehicle speed.